The invention relates to perforating devices for use in wells.
Perforating devices have been used by the oil-well service industry for many years to complete oil and natural gas wells. When wells are drilled into deep rock formations, they are cased to prevent the surrounding rock, sand, and water from invading the wellbore and interfering with the production of oil or natural gas. A typical casing material is high-strength steel pipe. In completing a well, a perforating device having an array of perforators (which may be shaped charge perforators) is lowered downhole into the well in a perforating gun. When the gun is at the correct depth in the well the perforators are fired, sending shaped charge jets outward first through the side of the gun, then through the fluid between the gun and the casing, through the well casing, and finally into the oil-bearing or natural gas-bearing rock. The resulting holes in the well casing allow the oil or natural gas to flow into the well and to the surface. What remains of the gun may be withdrawn from the well after the perforators have been fired.
The downhole formation adjacent the well may have many different characteristics. As examples, the formation may include competent rock that contains oil, gas or a loosely consolidated sand containing hydrocarbons. These types of formations govern the kind of perforators that are needed to complete the well. In the first case a perforator is needed that produces a large depth of penetration so that the maximum amount of rock is exposed to the hole in the well casing. In the latter case a perforator is needed that makes as large a hole in the well casing as possible so that gravel can be pumped through the hole to form a gravel pack, and depth of penetration is a secondary consideration. Penetrators used to create such large holes are sometimes referred to as big hole penetrators.
A shaped charge perforator may include a liner, a case to contain the liner, a high explosive, and some mechanism to initiate the detonation of the explosive. Typical materials for the case include steel or zinc. Typical liner materials include wrought materials such as copper, zinc, and various alloys or pressed powder including a mixture of copper, lead, and tungsten. An often used initiation mechanism includes a detonating cord that is positioned onto an opening at the rear of the perforator. Since the gun is typically withdrawn from the well after the perforators are fired, there is a constraint on the amount of explosive in the perforators. Furthermore, since perforators are used in large numbers every year, cost is a very important factorxe2x80x94both materials cost and manufacturing cost.
One way of manufacturing liners includes deep drawing a metal sheet into various shapes, such as conical, hemispherical shapes, and parabolic. Because ease of manufacture is an important consideration, these deep-drawn liners have approximately uniform thickness that approximates the uniform thickness of the original metal sheet. In order to be deep drawn, the liner material must be very ductile, so copper is often the material of choice. Other reasons for favoring copper are that copper has good penetration properties and copper is comparatively inexpensive.
In general, according to an embodiment, a perforating device for use in completing a well includes a case, an explosive charge contained in the case, and a generally bowl-shaped liner positioned adjacent the explosive charge and having non-uniform thickness along its length. The liner includes a protruding portion near its apex.
Other features of the invention will become apparent from the following description and from the claims.